Pressure sensitive copying paper is basically composed of a top sheet coated on its one side with microcapsules containing an oil solution of an electron donative colorless or light-colored dye, a bottom sheet provided on its one side with a coating layer containing an electron acceptant solid-state acid material and an adhesive, and an intermediate sheet coated on its one side with a microcapsule layer and on the other side with a layer made of an electron acceptant solid-state acid material.
It often occurs that the microcapsule layer and the electron acceptant solid-state acid material coated face come into contact with each other in the course of manufacture, printing operation, preparation of books, etc. For instance, such contact occurs when the intermediate sheet is rolled up or cut, or at the time of collation of sheets. In such a case, a certain degree of pressure or frictional force is exerted to the microcapsules, and as a result, part of the microcapsules might be broken to cause abnormal coloration or soiling, or so-called smudging, on the electron acceptant solid-state acid material coated surface. In order to prevent this, it is commonly practiced to mix in microcapsules a material having a greater particle size than microcapsules as a capsule protective agent (hereinafter referred to as stilt), and generally, fine powder of cellulose, starch granule and various types of plastic beads are used as such stilt.
The starch granules disclosed in Japanese Patent Publication Nos. 1178/72, 33204/73, etc., have fairly good properties in terms of smudging preventive effect, but since specific gravity of starch is large (1.6) in comparison with that of microcapsules (about 1) and also the particle size of starch used for said purpose is usually 20-30 microns, when the coating solution containing such starch is left still, there takes place sedimentation of starch granules to cause a change in composition of the coating solution in a continuous long-time coating operation, resulting in unfavorable variations of quality such as poor development of color on printing on the capsule coated face, smudging, etc. Also, defective color formation occurs when, for instance, taking a large number of copies due to deterioration of color forming performance in printing with weak impressing force owing to said size and hardness of the granules.
On the other hand, fine powder of cellulose, for instance, mechanically and chemically powdered and refined wood pulp, is widely distributed in particle size centering around about 20 microns in width, about 10 microns in thickness and about 100 microns in length, and also the specific gravity of such powdered cellulose is about 1.
Thus, in case finely powdered cellulose is used as stilt, there is no fear of sedimentation of particles in the coating solution because the specific gravity of cellulose powder is substantially equal to that of microcapsules, and therefore there occurs no change in composition nor any variation of quality even in a long-time coating operation. Further, such cellulose particles have proper hardness. Accordingly, when the paper surface is given an intentional pressing force such as applied for printing, there is little possibility that the break of microcapsules be retarded to impede color formation, and thus the paper shows good color forming characteristics even in printing with weak impression force. This type of stilt, however, because of its rod-like shape, proves in some cases unsatisfactory in its smudging preventing effect. Especially in the case of mononucleus microcapsules using amino resin as film material, although they are excellent in water resistance, solvent resistance, etc., the smudging preventive effect by the powder is sometimes found unsatisfactory in the system using cellulose powder.
Also, the top sheet of the system comprising mononucleus microcapsules using amino resin as film material and finely powdered cellulose may present the problem in long-time preservation stability, and the improvement thereof has been required.
Further, the system using said cellulose powder, when applied to the top sheet, may prove unsatisfactory in smoothness of the coated surface, and the improvement on such matter has been also desired.
The present invention has for its object to provide a pressure sensitive copying paper having excellent and well-balanced qualities such as water resistance, solvent resistance, long-time shelf stability, color forming performance, proofness against smudging, and smoothness.